This invention relates to web dryers and nozzles therefor which are used in the manufacture of paper and the like.
Numerous types of web dryers have been developed over the years, with the dryers utilizing a variety of types of nozzle assemblies. Representative assemblies are disclosed in the above identified patents, many of which use the Coanda effect as described in detail in U.S. Pat. No. 3,549,070. In U.S. Pat. No. 4,414,757, a nozzle assembly is disclosed which has a flat pressure plate adapted to form a gas flow zone with a moving web. A primary nozzle of the Coanda type is disposed at the upstream end of the pressure plate and continuously directs gas downstream along the face of the plate. A single secondary nozzle of the impingment type is disposed at the downstream terminus of the pressure plate, for reasons described in that patent.
Web dryers usually comprise a closed housing forming one or more web drying chambers or zones having a plurality of spaced parallel nozzle assemblies therein. The traveling web enters the housing through a narrow entrance slot, is acted on by air ejected from the nozzle assemblies, and ultimately exits the housing through a discharge slot. The working air is usually supplied from an outside source or sources, which normally heats the air and then passes through the nozzles into the drying zone and then exits through a suitable exhaust port.
Current web dryer technology generally requires that the housing interior be kept under a slight negative pressure, although positive pressure is also utilized in some instances. Under both circumstances, outside room air may tend to undesirably infiltrate into the housing through the web entrance and discharge slots. In addition, since the amount of pressure may vary from place to place within the housing, depending upon the location of obstructions (nozzles, for example) and the exhaust port, the infiltration of room air can be greater at one web slot than the other, causing air distribution imbalances.
For both positive and negative chamber pressures, the infiltration of room air is caused in part by the induction effect of the Coanda dryer nozzles, due to the flow of air around the curvature of the nozzles adjacent the web slots which in turn sucks air through the slots. This effect can be reduced somewhat by lowering the velocity of the jet flow through the nozzles. However, there is a lower limit to such velocity reduction, beyond which transient air currents inside the dryer chamber of air currents from adjacent nozzle jets will disrupt the desired air flow pattern.
In addition, sometimes the web may be narrower than the nozzle length, which may result in differences in flow from the nozzle assemblies between where a web is present and the web is absent. Also, it has been found that when cool room air infiltrates and contacts the nozzle parts that have been warmed by the heated dryer air, undesirable condensation may occur on the nozzle parts under certain conditions of dryer operation. Furthermore, infiltrating cool air tends to reduce the drying efficiency of the entire unit.
It is an object of the present invention to solve the aforementioned problems and to substantially and effectively reduce the infiltration of air from one zone into another in an efficient manner and at reasonable cost. It is a further object to reduce the effects of any small amount of air that may possibly still penetrate from one zone through a boundary into another zone.
In accordance with the various aspects of the invention, a unique nozzle assembly is provided for placement within one of a pair of adjacent ambient air zones and closely adjacent to the boundary therebetween. The nozzle assembly generally comprises a Coanda-type nozzle and a supplemental nozzle disposed on the assembly so that it is positioned between the Coanda nozzle and the zone boundary. Both the Coanda and supplemental nozzles are supplied with air from an air source means which in this embodiment comprises a common manifold connected to an external air source. An air flow control device is provided for the individual air flow paths in the assembly to suitably balance the velocities of the two discharging air jets.
In the disclosed embodiment, to prevent any transient air currents inside the dryer chamber from causing web or air flow instability, a seal is provided between the internally mounted improved nozzle and the dryer housing boundary wall. The seal is disposed along the head end of the nozzle; that is, closely adjacent the nozzle jet discharge ports.
In the disclosed embodiment, a pair of improved nozzles are usually disposed adjacent each housing web slot, one on each side of the web. The exact relative positioning of the nozzles in a pair may be varied according to the particular conditions encountered.
To overcome any problems caused by narrow webs or a slight amount of cool room air infiltrating the warmer dryer environment, a labyrinth of expansion chambers may be positioned between the inner dryer wall adjacent the slots and the improved nozzles. In the present embodiment, the labyrinth forms part of the nozzle assemblies themselves.